1. Field of the Invention
The invention relates to a pneumatic tire in which at least an outer periphery portion of the tire in a tread portion is formed of a non-conductive rubber layer, and a method for manufacturing a pneumatic tire.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects the fuel consumption of a vehicle and/or to increase braking performance (wet braking performance) on a wet road surface. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
There has been developed a pneumatic tire provided with a solution of the electric resistance problem in such a manner that a conductive layer blended with carbon black or the like is formed within a non-conductive tread rubber blended with silica or the like. For example, in pneumatic tires disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-71112, Japanese Unexamined Patent Publication (Kokai) No. 10-81110 and Japanese Unexamined Patent Publication (Kokai) No. 10-175403, a non-conductive tread rubber is formed with a conductive layer extending in a diameter direction of the tire to release the static charge to the road surface through the conductive layer. The conductive layer extends from a tread surface to a bottom surface through the tread rubber and is connected to conductive base rubber or sidewall rubber constituting a conductive path for releasing the static charge.
In a manufacturing process of such pneumatic tire provided with electric resistance solution, when the tire is subjected to a curing process, an end portion of the conductive layer to be exposed to the tread surface might be covered with a thin film of the non-conductive rubber in a so-called sheathed state. When such tire is practically used as it is, in an initial stage of wear, the releasing function of the static charge may fail causing a problem such as radio noises.
As a result of investigation, it was found that such sheath phenomenon tends to occur in a center portion, mediate portion and shoulder portion in the tread rubber in this order. The reason of this is understandable as described below. That is, in curing process of the tire, as cord angle of the belt layer changes due to the expansion of diameter of the tire, the tread rubber aggregates in the center portion. Since the diameter of the tire expands larger in the center portion, mediate portion, shoulder portion in this order, in the center portion where the diameter expands larger, the ratio of aggregation of the rubber is larger and tends to get in a sheathed state.
The above problem of sheath can be prevented by restricting the location where the conductive layer is exposed to the shoulder portion. In this case, such a request that the conductive layer be exposed in the center portion or mediate portion is not satisfied. Even when the conductive layer is exposed in a groove, the problem cannot be solved. Since selection of exposing location of the conductive layer depends on a pattern design somewhat, and in view of ensuring the flexibility of the pattern design, it is not desirable that the exposing location of the conductive layer is limited to a specific location.